Control mechanism including variable-stroke ram



R. H. COLLEY Nov. 28, 1961 CONTROL MECHANISM INCLUDING VARIABLE-STROKE RAM Filed July 14, 1958 3 Sheets-Sheet 1 N RM N N w Nov. 28, 1961 T R. H. COLLEY 3,010,434

CONTROL MECHANISM INCLUDING VARIABLE-STROKE RAM FiledJuly 14, 1958 s Sheets-Sheet 2 57 J10? awn? Nov. 28, 1961 CQLLEY 3,010,434

CONTROL"MECHANISM INCLUDING VARIABLE-STROKE RAM Filed July 14, 1958 3 Sheets-Sheet 3 United States Patent 3,010,434 CONTROL MECHANISM INCLUDING VARIABLE-STROKE RAM Rowan Herbert Colley, Derby, England, assignor to Rolls-Royce Limited, Derby, England, a company of Great Britain Filed July 14, 1958, Ser. No. 748,193 Claims priority, application Great Britain July 25, 1957 7 Claims. (Cl. 121--38) This invention comprises improvements in or relating to control mechanisms for moving an adjustable part and to uses of such control mechanisms.

According to the present invention, a control mechanism for moving an adjustable part comprises a ram cylinder, a plurality of pistons dividing the cylinder into a series of fluid spaces, the piston adjacent one end of the cylinder having a travel limited by a pair of fixed abutments and the remaining piston or remaining pistons having each a travel limited by a pair of corresponding abutments at least one of which is on the next adjacent piston towards said end of the cylinder, the piston adjacent the opposite end of the cylinder being the operating piston for the adjustable part, valve means controllable to supply pressure fluid selectively to the pressure spaces whereby the operating piston may be set in any of a number of positions predetermined by the abutments, power-operated adjustable abutment means adapted to be rendered operative and inoperative and, When operative, to cause the operating piston to be set at any desired position in a range of its travel, and selector means operable to render the power-operated adjustable abutment means operative and inoperative and to select the desired position in the range.

The control mechanism of this invention may be used with apparatus comprising the adjustable part which, under some conditions of operation of the apparatus, is required to be set in predetermined positions and, under other conditions of operation, is required to be set in any position within a range of its travel. During operation of the apparatus under the first condition, the poweroperated adjustable abutment is inoperative, and during operation under the second conditions the power-operated adjustable abutment is operative, the valve means being set to supply pressure fluid to the ram so that at least the operating piston follows the movements of the adjustable abutment.

The number of predetermined positions obtainable is dependent upon the number of pistons in the ram cylinder and upon the arrangement of the abutments for the pistons, a greater number of positions being obtainable when both abutments for a piston (apart from the first piston) are on the next adjacent piston than when only one abutment is on the next adjacent piston. For instance, when two pistons are employed three or four positions are obtainable according as one only or both abutments for the operating piston are on the first piston.

The range of travel under control of the power-operated adjustable abutment may be arranged to be any proportion of the maximum travel of the operating piston, and the adjustable abutment may co-operate with any of the pistons but preferably co-operates with the operating piston.

In a preferred construction of this invention, a control mechanism for moving an adjustable part comprises a ram cylinder, first and second pistons dividing the cylinder into end fluid spaces and an intermediate fluid space between the pistons, the first piston having a maximum travel limited by fixed abutments, and the second piston having a travel limited by a pair of abutments at least one of which is carried by the first piston, and the second piston being the operating piston for the adjustable part,

valve means controlling the selective supply of pressure fluid to said spaces whereby the operating piston may be set in any of at least three positions. predetermined by the abutments, power-operated adjustable abutment means co-operating with one of the pistons and adapted to be rendered operative and inoperative and, when operative, to cause the operating piston to be set at any desired position in a range of its travel, and a selector member adapted to render the power-operated adjustable abut ment means operative and inoperative and to select the positions of the adjustable abutment means in said range. In one such arrangement the second piston has one fixed abutment and one abutment on the first piston so that the second piston has three predetermined positions, and the adjustable abutment co-operates directly with the second piston and is arranged to move the second piston in a range of travel between an adjacent pair of the predetermined positions.

According to another preferred feature of this invention, the power-operated adjustable abutment means comprises an abutment member engaging the operating piston, a fluid-operated servo-mechanism which moves the abutment member relative to the ram cylinder, and control valve means operated by the selector member to render the servo-mechanism operative and inoperative and to select the setting of the abutment member, the pressure fluid supply to the ram cylinder spaces being controlled, when the servo-mechanism is operative, to maintain the operating piston in contact with the abutment member. In one particular arrangement according to this feature of the invention, the servo-mechanism comprises a piston working in a cylinder and having a piston rod one end of which projects into the ram cylinder to act as the abutment member, the cylinder of the servo-mechanism having operating fluid supply ports to each end thereof, and the control valve means for the servo-mechanism comprises a first valve controlling the supply of operating fluid through the ports and a second valve controlling the supply to the first valve to render the servo-mechanism operative and inoperative, these valves and the servo-mechanism piston moving in parallel paths, and there is provided a link, the ends of which are pivoted to the servo-mechanism piston and to the second valve respectively and an intermediate point of which is pivoted to the first valve, the position of the end of the link pivoted to the second valve being moved by the selector member. The selector member may be a pivoted lever having a range of angular movement, and it may be ar ranged for instance that the second valve prevents supply of operating fluid to the servo-mechanism (so that the adjustable abutment is inoperative) in a first portion of the range of angular movement and allows the supply of operating fluid (to render the adjustable abutment operative) in a second portion of the range.

A control mechanism according to the present invention has important uses in connection with gas-turbine engines. In such engines, especially when employed for aircraft propulsion purposes, it is often desirable to position accurately a part of the engine, such as a bleed valve, inlet guide vanes, thrust reversal devices and variable area nozzles, in accordance with some operating condition of the engine, for instance in accordance with compressor delivery pressure, pilots throttle setting and rate of delivery of reheat fuel.

According to one use of the control mechanisms, the operating piston is connected to adjust area-varying means of a variable-area jet nozzle at the outlet end of a jet pipe of the engine which has a reheat system in the jet pipe, the valve means controlling the selective supply of pressure fluid to the ram is arranged to be operable to provide a number of predetermined nozzle areas when reheat system is not being used and the selector member is adjusted by a pilots lever controlling delivery of reheat fuel to the jet pipe so that the nozzle is varied in a desired range in accordance with the rate of reheat fuel supply. In such a use of the control mechanism, the reheat fuel may be employed as operating fluid in the servo-mechanism above described.

One construction of control mechanism of this invention will now be described with reference to the accompanying drawings in which FIGURE 1 shows the control mechanism in a first 7 setting,

FIGURE 2 shows the control mechanism for range operation of the mechanism, and other features,

FIGURE 3 shows the control mechanism intermediate the ends of its travel in range operation,

FIGURE 4 is a diagrammatic view on a reduced scale of a gas turbine engine with a variable-area nozzle adjusted by an air motor.

The control mechanism shown in the drawings is suitable for use in connection with a gas-turbine engine having a jet pipe and a variable-area nozzle at the outlet of the jet pipe and also having reheat combustion equipment in the jet pipe. The control mechanism is arranged so that, when the reheat system is inoperative (FIGURE 1), the nozzle can be set to have any of three predetermined areas, viz: a minimum area, a maximum area and an intermediate area,-and so that, when the reheat system is operative (FIGURES 2 and 3), the nozzle area is increased in a range of areas between the intermediate area and the maximum area as the rate of delivery of reheat fuel is increased.

The control mechanism comprises a ram cylinder containing two difierential-area pistons 11, 12 dividing the cylinder into a number of spaces 13, 14, 15, 16 of which space 13 has a pressure fluid supply connection 17, the space 14 has a pressure fluid supply connection 18, the space 15 has a pressure fluid supply connection 19 and the space 16 is open to atmosphere through port 20.

The connections 17, 18 are connected to a supply drilling 21 through solenoid-controlled spring loaded valves 22,

23 respectively and the connection 19 opens directly into the drilling .21. The drilling 21 is shown as being supplied with pressure air tapped from a compressor 24 of the engine through conduit 25.

The valve 22 is loaded by its spring 22a so that when the associated solenoid 22b is de-energized (FIGURES 1, 2 and 3), the connection 17 receives pressure air from the drilling 21. The valve 2 2 has a bleed drilling 22c through it leading to a space 22d which, when the solenoid is energized, is open to atmosphere through port 26. Port 26 is closed by a valve extension 22c when the solenoid 22b is a de-energized.

The valve 23 is similar to valve 22 having a spring 23a solenoid 23b, drilling 23c, space 23d and extension 23c. When the solenoid 23b is de-energized, the spring 23a, urges the valve 23 to shut off the connection 18 from drilling 21 and to open the space 23a to a conduit 27. When the solenoid 23b is energized connection 18 is open to' drilling 21 and space 23d is cut 011 from conduit 27.

The conduit 27 contains spring loaded lift valve 28 having a small bleed orifice 29 and the conduit 27 is joined to a conduit 30 branching from the conduit 25. A shutoff cock 31 is provided in conduit 30 between its points of connection with conduits 25 and 27. The conduit 30 conveys pressure air to an air turbine 32 for driving a reheat fuel pump and when the reheat system is inoperative cock 31 is closed.

The piston 11 has its larger effective area facing space 13 and has its travel in the cylinder 10 limited by axiallyspaced travel-limiting fixed abutments afforded respectively by the end 33 of the cylinder 10 and by an internal shoulder 34.

ment 35 carried by the piston ll and by a second and axially-spaced travel-limiting fixed abutment formed by shoulder 36. The piston 12 has a piston rod 37 extending through piston 1'1 to outside the cylinder and the rod 37 is connected in any convenient manner to actuate an adjustable part which in the particular embodiment being described is the variable-area nozzle. For example as shown in FIGURE 4 the rod 37 of the control mechanism may control the operation of an air motor 101 for adjusting a variable area nozzle 102 at the outlet of a gas turbine engine 103. The piston 12 also has an axial skirt 38 which controls a port 39 the purpose of which will appear below.

In FIGURE 1 of the drawings, line A indicates the minimum area position of the nozzle, B indicates an intermediate area position and C indicates the maximum area position.

The three positions A, B, C are employed during nonreheat operation, and in such operation the cock 31 is closed so that pressure air does not reach air turbine 32 or conduit 27. The conduit 27 is, however, connected to atmosphere through the air turbine 32.

Position A.Solenoid 22b is energized and solenoid 23b is 1111-6I1Igl26d. Thus pressure air is fed solely to space 15, spaces 1'3, 14, 16 being at atmospheric pressure, and the pistons 11, 112 are moved to the right until piston L1 is against abutment 33 and piston 12 is' against abutment 35. This position corresponds to maximum engine r.p.rn., minimum nozzle area, no reheat.

Position B.-Solenoids 22b, 23b are un-euergized. Thus pressure air is fed to spaces 13 and 15, spaces 14 and 16 being at atmospheric. pressure, and piston 11 is moved to the left against abutment 34 carrying with it piston 12 which is against abutment 35. This position is employed under idling to cruise operation with no reheat and gives an intermediate area setting of the nozzle.

Position C.%olenoid 23b is energized and solenoid 22b is un-energized. Thus pressure air is fed to spaces 13, 14 and 15 and both pistons 11, 12 are held against their left-hand abutments 34, 36 respectively. This setting gives the maximum nozzle area and is employed during descent of the aircraft.

The control mechanism also comprises a power-operated adjustable abutment, which is employed during operation of the reheat system, to move the piston 12 to a position in a range of its travel dependent on the degree of reheat selected. a

The power-operated adjustable abutment comprises an abutment member 40 in the form of a rod projecting into the space 16 to co-openate with a projection 41 on the piston 12, a servo-mechanism for moving the rodand comprising a piston element 42 on the rod 40 working in a cylinder 43 having inlet and outlet porting 44, 45

' to the respective ends of the cylinder, and control valve means for controlling the supply of operating fluid to the porting 44, 45. Y

The operating fluid employed is reheat fuel under pres sure. The fuel enters the control mechanism through pipe 46 and flows into chamber 47 whence part flows to the servo-mechanism and the bulk flows to outlet pipes 48, 49, 50 leading respectively to pilot injector means, an inner ring injection means and outer ring injector means, the injection means being as usual in the engine jet pipe.

The fuel flows to the servo-mechanism through ducting 51 which has a first outlet 52 to a chamber 53 containing a pistonvalve 54 controlling the porting 44, 45 and a second outlet 55 to a chamber 56 containing a piston valve 57 which determines by uncovering and covering the outlet 55 whether pressure fuel is supplied to cylinder 43. When outlet 55 is uncovered pressure fuel cannot reach the cylinder -43 since the chamber 56 is connected via duct 58, drillings 59 in the valve 54,- and chamber 60 to a drain outlet 61. When outlet 55 is covcred pressure fuel reaches the cylinder 43 past the piston valve 54 and through porting 44 or 45.

The rod 40 has an extension 40a into chamber 60, and each of the piston valves 54, 57 has a stem 54a, 57a also projecting into chamber 60, and there is provided a link 62 which is pivoted at its ends to the extension 40a and the stem 57a and at an intermediate point to stem 54a. The end of link 62 which is pivoted to stem 57a is engaged by a forked end of a selector lever 63 pivoted in the casing of the control mechanism, and the lever 63 is moved in accordance with movements of the pilots control lever.

The pilots control lever has a range of movement in a gate from a position corresponding to idling of the engine to a position corresponding to maximum engine rpm. and has a further range of movement beyond the maximum r.p.m. position for reheat operation control. During reheat operation, the engine operates at its maximum r.p.m.

The ranges of angular movement of lever 63 are indicated at 64a (FIGURE 1) and 64b (FIGURE 2), the range 64a corresponding to non-reheat operation, and range 641; corresponding to reheat operation. The valve 57 takes up a position in its chamber 56 dependent on the angular setting of the lever 63 and the position of the outlet 55 is such that, when the lever 63 is in a position corresponding to maximum r.p.m. the outlet 55 is just covered and the outlet is also covered throughout the range of movement of valve 57 corresponding to the reheat range of movement 64b of the lever 63.

For reheat operation, cock 31 is open, solenoid 22b is un-energized, solenoid 23b is energized, and lever 63 is set in its range of travel 64b (FIGURE 2). Thus the reheat fuel pump driven by air turbine 32 is operating and pressure fuel is delivered to chamber 47, and pressure air is fed to spaces 13, 14 and 15 so that the piston 11 is against shoulder 34 and piston 12 is urged to the left.

With the lever 63 in the minimum reheat setting, i.e. at the right hand end of its range of travel 64b, the parts of the power operated adjustable stop take up the positions shown in FIGURE 2 and the piston 12 is held against the rod 40. This setting of the parts is obtained as follows:

Pressure fuel from chamber 47 flows through duct 51 and enters chamber 53 via outlet 52 (outlet 55 being covered), and, if the valve 54 is not in its equilibrium position as will normally be the case on selecting reheat operation, the fuel enters one end of the cylinder 43 via porting 44 or 45 to displace the piston 42 until it reaches the illustrated position (FIGURE 2) in which valve 54 covers both porting 44 and porting 45. Fuel draining from the opposite end of the cylinder 43 during this movement flow-s into chamber 60, either through porting 44 or through porting 45 and duct 59. Also since the piston 12 is urged by the pressure air in space 14 into contact with rod 40 and the end of the rod 37 takes up a position indicated in FIGURE 2 by line D giving a nozzle area suitable for minimum reheat.

It will be clear that on movement of the lever 63 from the position of FIGURE 2 to a new position in the range 64b, the piston 42 will be constrained to take up a new position within its cylinder 43, the operation being first that the valve 57 is moved to the right by the lever 63, as viewed in the drawings, carrying with it the piston valve 54 to uncover the porting 44, 45 permitting pressure fuel to enter the cylinder 43 through porting 45, and second that the piston 42 is moved to the left carrying with it the piston valve 54, this movement continuing until the porting 44, 45 is again covered. During this latter movement the valve 57 is stationary in the position set by the selector lever 63. Since the selector lever 63 has a range of move ment 64b corresponding to the desired degrees of reheat, the piston 42 will have a corresponding range of movement in its cylinder 43.

Further since, as has been described above, the piston 12 of the ram is maintained in contact with the rod 40, the piston 12 will follow the movements of the piston 42 and will take up a position withinthe cylinder 10 dependent upon the setting of the lever 63. Consequently the piston rod 37 will be set in a position so that its end is in a predetermined position in the range between the lines D and E, of which line E represents the maximum reheat setting of the nozzle. FIGURE 3 shows the parts set in a position intermediate the ends of the range D, E.

In the event that solenoid-controlled valve 23 fails to operate when its solenoid 23b is energized for reheat operation, a supply of pressure air will reach space 14 from conduit 25 through conduits 30 and 27 (cock31 being open during reheat operation and the valve 28 having a central orifice 29), space 23d, drilling 23c and connection 18. It is thus ensured that even if valve 23 fails to operate, piston 12 will be maintained against rod 40 during reheat operation.

The power-operated adjustable stop does not prevent movement of the piston 12 to the left into position C (FIGURE 1) during non-reheat operation, since when the maximum nozzle area is required in descent of the aircraft, lever 63 will be towards the right-hand end of its range of travel 64a and outlet 55 will therefore be uncovered and valve 54 cannot reach the position in which it closes off the porting 44, 45, so that piston 42 will be able to move freely to the left.

It will be clear from the foregoing description that there is provided control means by means of which the variable area nozzle may be set in any of three predetermined positions represented by lines A, B and C in FIGURE 1 or can be set at any position within a range of movement represented by the lines D, E (FIGURES 2 and 3) corresponding to the degree of reheat selected by adjust ment of the selector lever 63.

The control mechanism illustrated is also arranged to secure operation of a valve controlling the distribution of fuel to the various injection means of the reheat fuel system according to the setting of the selector lever 63 in the range 64b (FIGURE 2).

As will be seen from FIGURE 2, the fuel flows from chamber 47- past a non-return valve 70 into a chamber 71 containing a shuttle valve 74. Movement of the shuttle valve controls the distribution of fuel to the pipes 48, 49, 50.

Movement of the shuttle valve 74 is effected as follows. The shuttle valve has a pair of lands 74a, 74b joined by a reduced diameter portion 74c and also has a longitudinal bore 75 opening to its left-hand end, a cross bore 76 at its mid length opening into the bore 75 and a restricted bore 77 extending from the bore 75 to the peripheral surface of the land 74b. In all positions of the valve 74, the cross bore 76 receives fuel entering the chamber 71 past the non-return valve 70.

The end of the chamber 71 adjacent the land 74a is connected by a duct 78 to a further duct 79 one end of i which is connected through a restrictor -80 to duct 51 and the opposite end of which leads to a chamber 81 which is connected with the chamber '56 and with the chamber 53 containing the piston valve 54 so that the chamber 81 is open to the drain port 61. This end of the duct 79 is controlled by a half ball valve element 82 carried on a stem 83 connected to a flexible diaphragm 84 separating two spaces 85, 86 of which space is opento atmosphere through aperture 87 and ofwhich chamber 86 is connected to port 39. The chamber 86 also has a wall formed by a flexible bellows 88, one end of which is fixed to the wall of the casing of the control mechanism and the opposite end of which is secured to the stem 83 adjacent the half ball valve element '82. 7

When the piston 12 is in the minimum reheat area position of FIGURE 2, the pressure within the space 86 is atmospheric pressure and thus'the flexible bellows 88 holds the half ball valve element '82 firmly on the outlet from duct 79 preventing any outflow of fuel through the duct. Thus, when pressure fuel is being fed to the chamber 47, the pressure within the duct 79, which pressure acts on the right-hand end of the shuttle valve 74, will be substantially equal to the reheat fuel pressure upstream of the non-return valve 70, whilst the effective pressure acting on the shuttle valve 74 in the opposite direction will be. lower due to the fall of pressure across the non-return valve 70. Thus the shuttle valve 74 will be maintained in the position shown in FIGURE 2, in which the land 74a covers a port 89 leading to pipe 50 and fuel is supplied through the space around the reduced diameter portion 74c of the shuttle valve to the pilot fuel supply pipe 48 and through a port 90 to the fuel pipe 49.

When the piston 12 moves so far to the left that the port 39 is placed in communication with space 15, pressure air is fed from the space 15 to the chamber 86 so urging the diaphragm 84 to the right and lifting the half ball valve element 82 oh the outlet from the duct 79. This permits fuel to flow to drain through the duct 79 and, due to the restrictor 80, the pressure within theduct 79 falls to a value below the fuel pressure downstream of the nonreturn valve 70. The shuttle valve 74 is therefore moved to the right to a position in which the land 74a uncovers the port 89 leading to thefuel pipe 50, and in which the land 74b covers the port 90 leading to the fuel pipe 49 but the restricted bore 77 is opposite the port 90. In this way .a greater proportion of the reheat fuel flows to the pipe 50' and a minor propotrion flows through the cross bore 76 into the longitudinal bore 75 through the restricted bore 77 into the fuel pipe 49.

In this position of the shuttle valve 74, the pilot fuel pipe 48 is still supplied from the annulus around the reduced-diameter portion 74c of the shuttle valve. 7

Clearly as the degree of reheat selected by the selector lever 63 is reduced so the piston 12 will be moved to the right and will ultimately again cover the port 39 so that the bellows 88 will again expand closing the half ball valve element 82 onto the outlet from the duct 79 and when this position is reached the shuttle valve 74 will be moved back into the position of FIGURE 2.

I claim:

1. A control mechanism for moving an adjustable part, which mechanism comprises a ram cylinder, a plurality of axially-disposed pistons slidable in the cylinder and dividing the cylinder into a series of fluid spaces, a pair of fixed axially-spaced abutments co-operating with the piston adjacent one end of the cylinder to limit travel of this piston in the cylinder, .a pair of axially-spaced travellimiting abutments for each remaining piston at least one of which is on the next adjacent piston towards said one end of the cylinder, the piston adjacent the opposite end of the cylinder being the operating piston and having a part connectible to-the adjustable part, a source of pressure fluid, valve means connected between said source and the said fluid spaces and controlling the supply of pressure fluid selectively to the pressure spaces whereby the operating piston may be set in any of a number of positions predetermined by said pairs of abutments, an adjustable abutment co-operating with the operating piston and settable in any position in a range of movement thereby to set correspondingly the operating piston in position in said range, power-means connected to move the adjustable abutment in said range of movement and selector means controlling the power means, said selector means including a selector member having a setting in which said power-means is rendered inoperative to control the adjustable abutment and a further range of settings in which the power-means is rendered operative and sets the adjustable abutment in a position in said range of movement corresponding to the setting of the selector member.

2. A control mechanism according to claim 1, wherein the adjustable abutment co-operates directly with the operating piston.

3. A control mechanism for moving an adjustable part comprising a ram cylinder, first and second pistons sliding in the cylinder and dividing the cylinder into end fluid spaces and an intermediate fluid space between the pistons, a pair of axially-spaced fixed abutments co-operating with the first piston and limiting the travel of the first piston, a second pair of axially-spaced abutments co-operating with the second piston limiting the travel of the second piston, at least one of said second pair of abutments being carried by the first piston, and the second piston being the operating piston for the adjustable part, a source of pressure fluid, first valve means connected between said source and the said fluid spaces and controlling the selective supply of pressure fluid from said source to said spaces whereby the operating piston is movable into any of at least three positions predetermined by engagement of the pistons with the said abutments, an axiallyadjustable abutment cooperating with one of the pistons settable in any position in a range of movement thereby to set the operating piston in a corresponding range of movement, power means connected to move said adjustable abutment in its range of movement, and means controlling the power means including a selectormember having a setting in which the power means is rendered inoperative to control the adjustable abutment and having further a range of settings in which the power means is rendered operative and sets the adjustable abutment in a position in said range of movement corresponding to the setting of the selector member. 7

4. A control mechanism according toclaim 3, wherein the second piston has one fixed abutment and one abutment on the first piston so that the second piston has three predetermined positions, and the adjustable abutment cooperates directly With the second piston and is arranged to move the second pitson in a range of travel between an adjacent pair of the predetermined positions.

5. A control mechanism according to claim 3, said adjustable abutment comprising an abutment member engaging the operating piston, and said power-means comprising a fluid-operated servo-mechanism whichmoves the abutment member relative to the ramcylinder, and control valve means operated according to the setting of the selector member to render the servo-mechanism operative and inoperative and to select the setting of the abutment member, said first valve means controlling the pressure fluid supply to the fluid spaces, when the servo-mechanism is operative, to maintain the operating piston in contact with the abutment member.

6. A control mechanism according to claim 5, said servo-mechanism comprising a servo-piston Working in a servo-cylinder and having a piston rod, one end of which projects into the ram cylinder and acts as the abutment member, the servo-cylinder having operating fluid supply ports to each end thereof, and the control valve means comprising a first movable valve element controlling the supply of operating fluid through the ports and a second valve controlling the supply of operating fluid to the first valve element to render the servo-mechanism operative and inoperative, these valves and the servo-piston moving in parallel paths, a link having its ends pivoted to the servo-piston and to the second valve element respectively and having an intermediate point pivoted to the first valve element, the selector member being connected to move the end of the link which is pivoted to the second valve element.

7. A control mechanism according to claim 6, said selector member being a pivoted lever having two ranges of angular movement in the first of which the second valve element is set to prevent supply of operating fluid 'to the first valve element so that the servo-mechanism and adjustable abutment are inoperative, and in the sec- 0nd of which the second valve element is set to permit operating fluid to the first valve element so that the servomechanism and the adjustable abutment are operative, the position of the selector member in the second range determining the setting of the adjustable abutment in its range movement.

References Cited in the file of this patent UNITED STATES PATENTS Martin Nov. 10, 1925 Pelton June 18, 1935 5 Berry Apr. 23, 1946 10 Audemar Oct. 11, 1949 Jirsa June 6, 19'50 Hodgson Feb. 20, 1951 Poore Mar. 10, 1953 Colley July 7, 1959 Parker Oct. 13, 1959 

